What is the best method to keep bots from spamming your blog?

I got a problem at my blog. I got visits from kind bots who leave "nice" comments to my blog posts :(

I'm wondering if there is a smarter way to keep them out, besides using the captcha modules. My problem with the captcha modules is that I thinks they are anoying to the user :(

I don't know if it's any help to anyone but my site is in asp.net mvc beta.

How do I get rid of bot traffic?

The easiest way to keep bot traffic out of your Analytics reports is to use Google's automatic filter. To set up this filter, go to your view settings and check the box that says “Exclude all hits from known bots and spiders.”

Have you thought about using this?

http://akismet.com/

From their FAQ

When a new comment, trackback, or pingback comes to your blog it is submitted to the Akismet web service which runs hundreds of tests on the comment and returns a thumbs up or thumbs down.

It's a really easy to use system, which I highly recommend.